Replacing a Polycarbonate Roof: What You'll Actually Need

Polycarbonate roofing does not fail dramatically. It goes slightly cloudy, then a little more, and one spring you notice the space underneath is dimmer than it used to be and there is a green tinge in the flutes at one end. Nothing has broken. It has simply reached the end of its useful life, which for a good multiwall sheet is a long time but not forever.

Replacing it is one of the more satisfying jobs available on a veranda, because the frame it sits in is untouched and the improvement is immediate. It is also a job with a parts list slightly longer than people expect, and this is what is actually on it.

How to Tell It Is Time

Four signs, in roughly the order they appear.

Light transmission drops. This is gradual enough that you acclimatise to it, so the honest test is a photograph from the same spot a few years apart, or comparing the sheet at the sheltered end with the sheet that has taken the most sun.

Then discolouration, usually a yellow cast, which is ultraviolet degradation of the outer surface. Good multiwall sheet has a UV-protective layer on the weather face and it is the loss of that layer that starts the decline — which is also why fitting a sheet upside down is such an expensive mistake.

Then algae or dirt in the flutes, showing as green or grey streaking along the channels. Once it is inside the sheet it cannot be cleaned out, and it is a sign the end seals or the sealing tape have failed.

And finally brittleness. Old polycarbonate loses impact resistance, and a sheet that has gone brittle can crack when it is walked on or hit by a branch.

The Parts You Will Need Beyond the Sheets

This is the part that catches people out. Ordering only the sheets is ordering about three-quarters of the job.

Sealing tape comes first — the perforated anti-dust tape for the lower edge and solid tape for the upper edge of each sheet. It is not reusable, it is inexpensive, and it is what stops dust and insects getting into the flutes. Fitting new sheets with old tape is the reliable way to get green flutes again within two years.

Then the rubber seals in the beams and cover strips. These are colour-matched and pre-fitted from new, and after fifteen years of ultraviolet and compression they are hardened and taking a permanent set. They are cheap, they are the actual weather seal, and there is no sensible argument for reusing them once the roof is open.

Then cover strips and end covers, which take a beating during removal because they are the thinnest sections in the system and they were fitted by somebody who was not expecting to take them off again. Budget for a proportion of them.

Then the sheet stoppers or end profiles that close the ends of each sheet, and the screws. The supplied stainless screws in these systems come in specific lengths for specific jobs, and reusing screws that have been in for a decade and a half is a false economy.

Finally, a tube of the correct sealant. Note that the standard coloured sealant supplied with these systems is specified for aluminium to aluminium and to masonry, and is explicitly not suitable for sealing polycarbonate sheets — there is a separate transparent product for that, which matters if you have a downpipe passing through a sheet.

Do the Rest of the Structure While It Is Open

With the roof off, several jobs become five-minute tasks that are otherwise awkward.

Clean the gutter properly, including the outlet and the leaf catcher, and check the fall to the downpipe with a hose. Check every fixing on the structure and nip up anything that has worked loose, which is exactly what the manuals ask you to do regularly and almost nobody does. Inspect the wall junction sealant along the top edge of the wall plate and renew it if it has hardened or lifted. And look at the gable panel at the same time if you have one, since it is glazed into the same system and the same seals apply.

This is also the moment to reconsider the finish. If the original roof was clear and the space has always been too bright and hot, opal or solar control sheet costs the same to fit and changes the space completely.

Getting Longer Out of the New One

Three habits genuinely extend the life of a polycarbonate roof.

Fit it the right way up. The UV-protected face is marked and it goes outwards. This sounds too obvious to state and it is the most common cause of a roof that fails early.

Keep the flutes drained and taped. The tapes are doing a real job, and a cut edge left open is an invitation.

Clean it with plenty of water and something soft. The manuals specify neutral cleaners only, no acids or alkalis, and solvents such as washing-up liquid for greasy marks. Abrasives and harsh chemicals attack the surface layer, which is the layer doing the protecting.

What It Costs Against the Alternative

Re-sheeting is a fraction of the price of a new structure, because the frame — the expensive, engineered part — is untouched. The aluminium will comfortably outlive two or three roof coverings, which is precisely the argument for buying a decent frame in the first place.

Doing It Yourself, Realistically

Re-sheeting is one of the more approachable jobs on a veranda, and it is still work at height with awkward materials.

Multiwall sheet is light, which sounds helpful and means a full sheet behaves like a sail in any breeze — this is a still-day job with two people, not a windy Saturday alone. You are working above head height throughout, so the ladder or tower matters more than the tools. And the old sheets have to come down and go somewhere; polycarbonate is recyclable but is not a kerbside item, so establish where it is going before you start rather than living with a stack of it against the fence for six months.

One more thing worth doing while the roof is off: check the beam-to-gutter and beam-to-wall connections. These are the joints carrying everything above, they are completely inaccessible once the sheets are back on, and a fixing that has worked a quarter turn loose over fifteen years is a two-minute job at this moment and an awkward one at any other. The manuals ask for fasteners to be checked regularly; in practice a re-sheet is the only time most people ever get the opportunity.

It is also the natural moment to ask whether the roof could be glass instead. Sometimes it can, and sometimes it cannot, because it depends on whether the model was built with a glazing channel and a gutter able to take the reinforcement that glass requires. That is a specific question with a specific answer, and it is worth asking before you order sheets rather than afterwards. If the answer is no, a new polycarbonate roof in a better finish is a genuinely good outcome — and a matching side panel or two while the scaffold-free access is there costs very little extra.